The removal of suspended solids from a water supply is commonly accomplished with several types of sloped plate water clarifiers. In a typical design, the untreated water or slurry is directed into a tank that contains a plurality of parallel, spaced apart plates that are inclined with respect to the horizontal. As the individual particles in the slurry contact the sloped plates, the particles coalesce into larger groups on the surface of the plates. When these groups are heavy enough to lose their buoyancy, they dislodge from the plates and sink downwardly to settle in the bottom of the tank. The clarified water rises in the tank and is discharged near the top. The patents to Larsson et al. U.S. Pat. No. 4,400,280 and Lee U.S. Pat. No. 4,664,802 are examples of sloped plate clarifiers having this basic design.
One area of improvement needed for these clarifiers is in the positioning of the sloped plates inside the tank. Also, in most prior arrangements, the plates are fixedly attached at a particular angle. With this arrangement, the settling process cannot be controlled by varying the slope angle of the plates. This limits the ability of the operator to adjust for changing slurry conditions and operating requirements.
Another problem with the clarifiers of the prior art is that they provide no means for separating and recovering valuable process product from the impurities and other suspended solids in the slurry. In the prior art designs, the untreated slurry is introduced directly into the clarifier tank and any process product in the slurry is collected at the bottom of the tank with the other residue and expelled. Particularly in a quarry or mining operation, the amount of product lost in the clarification process is thus often substantial.
An inherent limitation with all sloped plate clarifiers is that the natural settling process provided can only achieve a certain level of clarification. As the size of the suspended solids in the slurry decreases, the settling characteristics of these solids become poor and the effectiveness of the sloped plate clarifier correspondingly decreases. At a certain point, the desired clarity may be unobtainable or require an unacceptably slow flow rate.
To counter this problem, some clarifiers incorporate chemical flocculants that agglomerate the smaller particles into larger groups that are more easily settled. The most commonly used flocculants are water-soluble copolymers having a very high molecular weight. These polymer flocculants are typically used in a liquid form that is mixed with the untreated slurry at the beginning of the clarification process.
Although the use of polymer flocculants increases the effectiveness of sloped plate clarifiers, it is also a relatively expensive process. Not only is the cost of the polymer flocculant itself substantial, but the capital cost of the equipment required for feeding a liquid flocculant into the system is very expensive, and even considered by some to be cost prohibitive. That is, these systems require a separate tank for storing the liquid polymer and a separate pump station with relatively expensive electronic controls to regulate and monitor the flow of the polymer into the clarifier. In addition, a liquid polymer flocculant system is maintenance intensive and requires regular human intervention and adjustment to achieve the proper level of clarification. An example of a sloped plate clarifier that uses liquid polymer flocculant is U.S. Pat. No. 5,143,625 to Ballard.
A recent development in the field of polymer flocculants is the availability of a concentrated solid form, generally referred to as a polymer gel log. These logs are placed directly in the untreated slurry and gradually dissolve to provide controlled release of polymer flocculant. Coagulation of the suspended solids begins immediately downstream of the logs and settling and precipitation of the suspended solids is substantially improved. An example of this form of polymer flocculant is the Photafloc polymer gel log manufactured by Neutron Products, Inc., Dickerson, Md. 20842.
Using polymer gel logs provides several distinct advantages over liquid polymer. Most importantly, it has been found that by using polymer gel logs in combination with a mechanical mixer to generate turbulence in the flow inlet passageway, the consumption of polymer flocculant is dramatically reduced while still maintaining an effective level of supply to promote settling of the solids. For example, in a stone washing process in a limestone quarry the cost of treating the slurry or washwater with polymer gel logs is determined by testing to be approximately one-tenth the cost of treating the same amount of slurry with liquid polymer. Additionally, using polymer gel logs eliminates the need for separate storage tanks and electronically controlled pumping stations that are required for dispensing the liquid polymer. Polymer gel logs require only turbulent contact with the untreated slurry, which is easily provided by simple mechanical mixing. Finally, the mechanical mixer needed for polymer gel logs requires very little maintenance and much less human intervention than the equipment used in a liquid polymer system. It has been discovered that the polymer dissolution of the logs automatically adjusts according to changing flow conditions, thereby requiring only occasional observation and intervention.
Thus, as demonstrated by the deficiencies of the prior art, there is a need identified for a sloped plate water clarifier that has an improved plate arrangement including adjustable plate angles in at least one section for increased control over the settling process. In addition, a better approach for separating and recovering useable process product that is mixed with the untreated slurry has long been sought, but still remains unsolved in so far as I am aware prior to the present invention. Also, a workable mechanical mixer to be used with polymer gel logs to provide cost-effective and more efficient flocculant treatment is desired.